According to conventional technology, highly precise, wear-resistant chains, especially those having little stretch and no slack, are often formed by the use of chain link assembly bodies which include a sleeve inserted into a bushing. The current state of the art is defined by Japanese Laid-Open Patent Application Publication No. 5-278064 (A) to Koji Tanaka ("Tanaka") which teaches the manufacture of an inner link assembly having a resin sleeve over the peripheral surface of a bushing of a bushed chain.
FIGS. 1(A)-1(F) illustrate the method of the Tanaka reference. Referring to FIG. 1(A), Tanaka teaches a cylindrical roller R which is immersed in an electroless nickel bath B1 so that a thickly plated layer N is formed on the entire surface of the cylindrical roller. FIG. 1(B) shows the thickly plated cylindrical roller R loosely fitted outside a cylindrical bushing 2 and both of the end parts of the bushing 2 are press-fitted into the connecting pin insertion holes 1' of the inner links 1. FIG. 1(C) shows both of the end surfaces of the cylindrical roller R which are snugly put into press-fitting contact with the inside surfaces of the inner links 1, a cylindrical space .delta. defined between the inner peripheral surface of the cylindrical roller R and the outer peripheral surface of the bushing 2, and an injection hole P for press-injecting resin into the space .delta. made penetratingly in the side of the inner link. In FIG. 1(D), the inner link assemblies are inserted into a mold M and, following this, heat-melted resin is poured into the cylindrical space .delta. from the injection hole P by injection molding.
The Tanaka reference states its purpose as to avoid the occurrence of deflected abrasion and hence, increase the abrasive life of the sleeve by press-fitting a bushing into the connecting pin insertion hole of an inner link for making them up into an inner link assembly and thereafter by forming molten resin integral with the bushing peripheral surface through injection molding.
The method taught by Tanaka has the problem of producing a swelling stress on the bushing from the formation of the bushing's convex contour by expansion. The injection molding of the resin into the bushing creates a further swelling stress due to the injection pressure at the time of press-fitting the bushing into the coupling pin insert holes of the inner links. Upon leaving the mold, a restorative stress is produced which attempts to return the bushing to its original unexpanded state. Thus, creep destruction is introduced to the bushing which considerably decreases the durability of the bushing.
The present invention solves the problems of the prior art by providing an improved chain link assembly body having a more durable bushing and a method of forming a chain link assembly body wherein the bushing need not be expanded in order to eliminate possible creep deformation of the bushing. The method includes providing a pre-molded resin sleeve having a uniformly cylindrical inner periphery and an partially convexly contoured outer periphery which is inserted into a bushing wherein prior to insertion the bushing may be uniformly cylindrical in cross-section due to not having either of its ends press-fitted into coupling pin insert holes of inner links (first embodiment), non-uniformly cylindrical in shape due to having one of its ends press-fitted into the coupling pin insert hole of an inner link (second embodiment), or partially convexly contoured due to having both of its ends press-fitted into coupling pin insert holes of inner links (third embodiment).
According to the present invention, a resin sleeve is molded into a uniformly cylindrically shaped inner periphery and a partially convexly contoured outer periphery prior to being inserted into a bushing. Thus, when the sleeve is pre-molded, there is no injection pressure imposed on the bushing because there is no need for injection molding of the resin sleeve. Therefore, there is no restoring pressure attempting to return the bushing to its originally unexpanded and non-convexly contoured state and there is no creep deformation stress imposed on the bushing.
It is an object of the present invention to provide an engineering plastic resin sleeve, which has been molded into a shape having an uniformly cylindrical inner periphery and an outer periphery having a convexly contoured central portion and two opposed, uniformly cylindrically contoured end portions prior to the sleeve's insertion into the bushing. The pre-molded sleeve is elastic enough so that when inserted into the bushing, the bushing is not expanded as was the case in the prior art. Thus, there is no swelling pressure imposed on the bushing by insertion of the sleeve and the bushing will not be subjected to any restoring stresses attempting to return the bushing to its original state from an expanded state so that creep deformation is eliminated. This will, in turn, prolong the life of the bushing by maintaining the bushing's durability.